If you are going for ND & ND Grads then I would personally reccommend staying away from Cokins. In my experience and others they have a magenta cast as they aren't true NDs just a grey filter. Although in the interests of ballance it may be that I had a bad experience with poor quality - they are french :razz: after all
I know the Lee Filters are much more expensive but I can honestly say they are worth every penny plus you won't have a Vignetting problem as they are 100 x 150mm (for the grads). Alternatively if you can find them Hi-Tech filters are reccommended as a suitable alternative by those on the web who have had similar problems to me (see link 2 IIRC"]
I could go on but I'm bored now